These are Holographic Diffraction Gratings, highly efficient embossed Holographic Optical Elements (H.O.E.).Diffraction Gratings are used for the direct viewing and analysis of spectra from different gas tubes and other light sources.The quality of the spectrum produced from these gratings is the brightest possible with a minimum of distracting visual noise.These gratings are the principal component in a spectroscope and are used for experiments pertaining to the study of light and colour.Project a spectrum using an overhead projector or 35mm slide projector for spectrum demonstration purposes.Use in conjunction with coloured film gels for additive and subtractive colour demonstrations.2" X 2"